The present invention relates to a method for the operation of an optical emission spectrometer.
Emission spectrometers with spark and/or arc excitation are used in the multi-element routine analytics of metals. FIG. 1 shows the general prior art on the basis of a diagrammatic representation of the structure of such systems. The stand (1) allows for the mounting of a sample (2) at a distance of 0.5 to 5 mm from a counter-electrode (3). The excitation generator (4) firstly creates a high voltage pulse, which ionizes the atmosphere between the sample surface and the counter-electrode (air or inert gas) and so renders it of low resistance (low-Ohmic).
With the arc generator, a direct current of strength 1 A to 10 A is then fed via the low-resistance spark path; this arc is sustained for a period from 0.5 s to 10 s. Arcs of this nature are mostly operated in an air atmosphere.
Instead of one single long pulse, a spark generator generates short pulses of a duration of 50 μs to 2 ms with a sequence frequency of between 50 Hz and 800 Hz. A new ignition pulse is required before each individual spark. A thermal plasma is formed with temperatures of between 4,000 K and 20,000 K, in which free atoms and ions are excited for the emission of a line spectrum. The emitted light is conducted into an optical system (5), on the focal curve (6) of which the spectral lines are sharply formed. The spark excitation takes place as a rule in an argon atmosphere.
At the present time there are two conventional methods for measuring the spectral lines sharply imaged on the focal curve.
1. The first type of spectrometer system is shown in FIG. 2, which also represents the prior art. The light impinges through a source slit (7) onto a concave grating (8). A spectrum occurs as a plurality of wavelength-dependent diffraction patterns of the source slit. The spectral lines of interest are masked out with exit slits (9) and their intensity is measured by means of photomultiplier tubes (10).
2. The second conventional form of spectrometer design according to the prior art is shown in diagrammatic form in FIG. 3. With this design, too, the light falls through a source slit (7) onto the grating (8). However, instead of an individual exit slit, multi-channel sensors (11) are arranged here along the focal curve (6). These multi-channel sensors consist of a linear arranged field of photo-sensitive sensor elements, referred to as pixels. In this design, a simultaneous absorption of complete spectral ranges is possible.
The conventional calibration of the spectrometer systems now takes place in such a way that the totality of the materials to be analyzed with the system are subdivided into material groups of similar chemical composition. If it is intended, for example, that a spectrometer system should measure all materials which consist predominantly of iron, such groups are low-alloyed steels, cast irons, manganese steels, chrome steels, and chrome-nickel steels.
For each of these material groups combinations of analyte lines and lines of the basic element are known (iron in the example referred to), which are particularly well-suited for setting up a calibration function. The lines of the basic element (referred to as the internal standards) serve to compensate changes in the plasma. They are individually selected to suit each analysis line. The calibration function of an analysis line is determined first by a set of standard samples being measured for a given group of materials. Next, the intensity ratio for each sample (quotient of the measured value of the analysis line divided by the measured value of the internal standard pertaining to it) is applied against the concentration ratio (concentration of the analyte divided by the concentration of the basic element). Finally, a polynomial is determined over these value pairs (each value pair is the tuple (concentration ratio, intensity ratio)) by means of regression calculation with which the sum of the square deviations between the polynomial and the sample concentration ratio is minimal. In the simplest case, the polynomial which is found is the calibration function which is sought. It is often necessary, however, for the influences of third elements to be taken into account in the regression calculation. The performance of this calculation is described, for example, in Slickers [K. A. Slickers: Automatic Atom-Emission Spectral Analysis, Brohische Universitatsdruckerei, Giessen, 1992]. The standard deviation of the deviations between calibration and sample concentration ratio is designated as scatter residual (abbreviated to SR). Suitable calibration functions are characterised by a low scatter residue.
If precise analyses are to be carried out of metals from material groups with sharply varying contents of alloy elements, the electrical spark is the method of choice. Combinations of analysis lines and internal standards can be found of which the scatter residue is perceptibly lower than that of the best line pairs known with arc excitation. It is also to be pointed out that with arc excitation and calibrations of material groups with sharply varying element contents no line pairs can be found and the variation coefficient of the intensity ratios is, as a rule, unsatisfactory. It often lies at between 10% and 50%, in comparison with the typical 3-10% with arc excitation and alloy groups with low concentration variations, and 0.1-3% for spark calibrations.
The good accuracy and high precision of the spark excitation are obtained at the expense of certain disadvantages:                The use of Ar flushing with typical flushing rates of 2 l/min during the measurement requires that a voluminous and heavy pressure cylinder be carried with the system, which renders portable systems impractical.        The spark excitation requires a clean ground flat surface. With heavily dirt-contaminated or oxidised surfaces, there will be no or only irregular material decomposition.        A spark measurement typically lasts for 15 s as opposed to typically 3 s with arcs.        The spark opening must be sealed against the surrounding atmosphere during the measurement. The penetration of air impairs the measurement. Spark apertures of 4 to 20 mm are usual. It follows from this that only samples can be measured which are provided with a flat surface of the given size.        
Excitation with an arc is therefore substantially easier to carry out, in particular with portable spectrometers.